Every now and again, [T-Kuhn] says that the ball will bounce in an unexpected direction ... because there is so much going on at once; you can check it out just below the page break.

[Eric Archer] constructed an analog computer to model the physics of a bouncing ball. The core is a TL074 opamp that does all the integral math. He had no trouble finding descriptions of analog ...

You can often see the effect better when the balls drop just half a metre or so. This stunt relies on the lower ball bouncing up and hitting the upper ball while it's still on the way down.